Our goal is to work out with the electron microscope the intimate details of synaptic connection in various parts of the cat visual system and to correlate this finding with the physiological responses of the neurons. Our current approach is to recognize particular types of synapses, either by virtue of their normal morphology, or by labelling them chemically or causing them to degenerate. The postsynaptic profile contacted by these synapses are then followed in serial sections until they can be identified with a particular cell type. Information on the synaptic "context" of the labelled synapses is gathered at the same time. At present we are working on the superficial gray layer of the superior colliculus and area 17 of the visual cortex.